The present invention relates to a focus control device for use in an image pickup device such as a still camera or the like, for controlling the focused state of an image produced by a lens unit, a focus control method for controlling the focused state of such an image, and an image pickup device incorporating such a focus control device therein.
Almost all image pickup devices such as stand-alone digital still cameras, digital still cameras combined with cellular phones, etc. are constructed such that image light from an image capturing lens is focused onto an imager such as a CCD sensor or a CMOS sensor to generate an image signal. Most such image pickup devices have an automatic focusing function. The automatic focusing capability generally refers to an automatic process for controlling the position of a focusing lens to keep the image focused without the need for a manual focusing process. One automatic focusing process which is well known in the art is a contrast method which utilizes the contrast information of an image signal generated from image light from the image capturing lens.
Japanese patent No. 3401847 discloses a circuit for performing an automatic focusing process based on the contrast method. According to the disclosed automatic focusing process, the focusing lens is moved in a focusing direction depending on an evaluated value which corresponds to the contrast of the image signal. The evaluated value is acquired and the movement of the focusing lens is accordingly controlled in synchronism with a vertical synchronizing signal. Even in image pickup devices such as digital still cameras which mainly handle still images, most of processes carried therein including a signal processing process, a camera controlling process, etc. are normally performed in timed relation to one field (frame).